Introduction

Total hole depth is the maximum bit depth. It is different from true vertical depth (TVD) by taking into account of deviated wellbore length. Typical unit for bit depth is feet for field unit or meter for SI unit. In our approach, measured depth is estimated by using block height data to estimate length of drill string within borehole, without taking into account of drill string elongation or other effect that could introduce depth errors. Measured depth will either increase or stay constant, and is only calculated during drilling activity. Rig state information is required for this calculation. Measured depth is often used for annular/drill string volume, true vertical depth during deviated wells, and other drilling calculations.

Calculation Approaches

The first step to calculate measured depth is to obtain block height data. In this approach, measured depth is estimated by accumulate block height changes during drilling activity only.
The equation for calculating bit depth is:

MeasuredDepth_feet(i) = MeasuredDepth_feet(i-1)+(blockHeight_feet(i-1)-blockHeight_feet(i))

where i is the number of data entry. Note measured depth will either increase or stay constant for each new data entry. The length of the wellbore will never decrease.

Metadata Required

• Block Height – depth change of drill strings with corresponding time of recording.

• Rig State – calculation only performed in certain rig state activities

References

Poletto, F. B., and Francesco Miranda. Seismic while drilling: Fundamentals of drill-bit seismic for exploration. Vol. 35. 2004. Adam, T. B., et al. "Applied drilling engineering." SPE Textbook Series, Dallas, TX 2 (1991).